Rename "fallback" recipe attribute to "default_value"

Problem/Motivation

While naming things is one of the two hardest things in software development, the problem can be greatly simplified by reusing existing naming conventions.

In ✨ Add default value for config source on input defaults for recipes Active a new attribute was added to the recipe system for providing a value that is to be used should another value not be available. Rather than using a similar name for this attribute to what is used in the migration system, i.e. "default_value", the name "fallback" was used. This makes recipe development marginally harder as the naming convention that a developer might have used in another subsystem is not used here.

Proposed resolution

Rename the "fallback" attribute to "default_value".

Remaining tasks

Rename the "fallback" attribute to "default_value".
Update tests.

User interface changes

n/a

Introduced terminology

It removes the use of "fallback" terminology for describing a default value.

API changes

n/a

Data model changes

n/a

Release notes snippet

All uses of "fallback" in recipes to describe a default value must be replaced with "default_value".